1. Field of the Invention.
This invention is in the field of computer generated raster graphics and more particularly relates to a method and apparatus for filling polygons displayed by a color CRT monitor of a computer generated raster graphic system.
2. Description of the Prior Art.
Raster scan CRT displays form a principal communication link between computer users and their hardware/software systems. The basic display device for computer generated raster graphics is the CRT monitor, which is closely related to the standard television receiver. In order for the full potential of raster graphics to be achieved, such displays require support systems, which include large-scale random access memories and digital computation facilities. As the result of recent developments, particularly of large-scale integrated circuits, the price of digital memories has been reduced significantly and computers in the form of microcomputers are available which have the capability of controlling the displays at affordable prices. As a result, there has been a surge of development in raster graphics. Typically, each pixel in a rectangular array of picture elements of a CRT is assigned a unique address, comprising the x and y coordinates of each pixel in the array. Information to control the display is stored in a random access memory (RAM) at locations having addresses corresponding to those assigned to the pixels. The source of pixel control data written into and stored by the RAM is typically a microcomputer located in a graphic controller which will write into the addressable memory locations the necessary information to determine the display. This information frequently includes an address in a color look-up memory, at which location in the color look-up memory there is stored the necessary binary color control signals to control the intensity of the color of each pixel of an array. The horizontal and vertical sweep of the raster scan is digitized to produce addresses of pixels, which addresses are applied to the memory in which the controller has previously written the information determinative of the display; i.e., the color and intensity of the addressed pixel as it is scanned in synchronism with the raster scan. The data stored in the addressable locations of the color look-up memory is read out of the addressed location in the color look-up memory and the necessary color control signals are obtained. The color control signals are converted to analog signals by digital to analog circuits and the resulting analog signals are applied to the three color guns of the typical CRT to control the intensity and color of each pixel as it is scanned.
Raster graphic systems having the capability of displaying polygonal shapes which are filled with color are known. The most relevant information concerning such techniques for filling polygons is found in an article by Bryan Ackland and Neil Weste, "Real Time Animation Playback on a Frame Store Display System", Computer Graphics, Quarterly Report of SIGGRAPH-ACM (July, 1980), pp. 182-188. One problem with prior art polygon fill techniques is that such techniques require a large amount of I/O activity between the graphic controller and the frame memory, which, of course, limits the capability of the graphic controller to do other things. A second problem is that an ambiguity occurs when the boundaries of a polygon intersect the same pixel of a horizontal scan line. As a result, special software programs are required by the graphic controller to prevent the system from continuing a polygon color fill element beyond the intersection. To describe the ambiguity in other words, how does one handle a situation in which the length of a fill element is one pixel.